2. Computers and Programs Zelle - Chapter 1 Charles Severance - www.dr-chuck.com Textbook: Python Programming: An Introduction to Computer Science, John Zelle

3. Users.vs. Programmers Users see computers as a set of tools - word processor, spreadsheet Programmers have some tools that allow them to build new tools Programmers sometimes write tools for lots of users and sometimes programmers write little widgets for themselves to automate a task

4. Why do we program? To get some task done - part of some non-programming job Clean up survey data To produce something for others to use - a real programming job Fix a performance problem in the Sakai software

5. Computer Hardware + Software Networks.... From a software creator’s point of view, we build the software. The end users (stakeholders/actors) are our masters - who we want to please - often they pay us money when they are pleased. But the data, information, and networks are our problem to solve on their behalf. The hardware and software are our friends and allies in this quest. InformationData User Creators (Screenshot) Source: ctools.umich.edu (iPhone) CC BY: Johan Larsson (flickr) http://creativecommons.org/license/by/2.0

6. What is Code? Software? A Program? A set of stored instructions It is a little piece of our intelligence in the computer It is a little piece of our intelligence we can give to others - we figure something out and then we encode it and then give it to someone else to save them the time and energy of figuring it out A piece of performance art

7. How Many Lines in a File? You could read and understand Chapter 4 page 110 Chapter 11 Page 341 Appendix A Page 447 Or I could send you this in E-Mail: infile = open("mbox", "r") print len(infile.readlines())

8. Hardware Architecture

9. Software Software InputDevices CentralProcessingUnit MainMemory OutputDevices SecondaryMemory Generic Computer Z-5

10. Definitions Input Devices: Keyboard, Mouse, Touch Screen Output Devices: Screen, Speakers, Printer, DVD Burner Central Processing Unit: Runs the Program - AKA The CPU is always wondering “what to do next”? Not the brains exactly - very dumb but very very fast Main Memory: Fast small temporary storage - lost on reboot - aka RAM Secondary Memory: Slower large permanent storage - lasts until deleted - disk drive / memory stick

11. Hardware Hardware Software Software InputDevices CentralProcessingUnit MainMemory OutputDevicesSecondaryMemory Network/Internet (iPhone) CC BY: Johan Larsson (flickr) http://creativecommons.org/license/by/2.0

12. Hardware Hardware Software Software CentralProcessingUnit MainMemory SecondaryMemory Network/Internet Web Server A web server often functions with no input or output devices connected to the system. It takes incoming requests from the network - does some work with those requests and send output back across the network. (Servers) CC BY: Jesse Wagstaff (flickr) http://creativecommons.org/license/by/2.0

13. Network/Internet Hardware Software (Servers) CC BY: Jesse Wagstaff (flickr) (Phone) CC BY: John Larsson (flickr) http://creativecommons.org/license/by/2.0

14. Programmer Tools

15. Becoming a Programmer We use the computer - we just have to learn some programmer tools Compiler - Takes our code and makes it executable Interpreter - Reads our code and runs it Development Environment - Helps us write code

16. When a Program Runs... When a program runs it: Takes some input data Processes the data using a set of instructions (a program) Produces some output Think of it as “value add” An example program takes a text file as its input and counts the lines in the file and prints out the number of lines in the file. Z-8

17. Programmer Tools We use the computer - we just have some new tools Development Environment - A “Word Processor” or “Text Editor” for Programmers - we write code in a development environment Compiler - Takes our code and makes an executable version of our program Interpreter - Reads our code and runs it directly - Python is an interpreted language - Python is an interpreter

18. Z-8 User A programmer develops a program. If a compiler is used the compiler translated the source to machine code for distribution. If an interpreter is used, the programmer simply distributes the source code. Data Programmer Data User

19. Terms Source code - the programs we humans write - and read - written in a programming language - source code is generally portable across systems Machine code - what really runs on the machine - not very readible - produced by a compiler - machine code is unique to hardware and operating system.

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23. Z-8 User A programmer develops a program. If a compiler is used the compiler translated the source to machine code for distribution. If an interpreter is used, the programmer simply distributes the source code. Data Programmer Data User

24. Compiler.vs. Interpreter Only the programmer needs to have the compiler - once the compiler is done - the executable program is self-contained The programmer keeps the source code and distributes the executable - different executables are needed for Mac, PC, etc. Both the programmer and user need to have the Interpreter installed on their system Generally the programmer distributes the source code of the program

25. Python is an Interpreter To run Python programs, users must install Python on their computers Development is quick and easy - we simply make a change to our program and run it again in a single step For data analysis - Python is just a tool that you keep on your desktop or laptop Interpreters are more convenient when the user and programmer are the same person

26. Running Python Interactively Z-9

27. Python Interactive Since Python is interpreted we can just type programs directly into Python See Also http://datamech.com/devan/trypython/trypython.py x=1 print x x = x + 1

28. csev$ python Python 2.5 (r25:51918, Sep 19 2006, 08:49:13) [GCC 4.0.1 (Apple Computer, Inc. build 5341)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> x = 1 >>> print x 1 >>> x = x + 1 >>> print x 2 >>> exit() This is a good test to make sure that you have python correctly installed.

29. Syntax Errors The computer has a language where you an tell it what you want to do - this is Python It seems unfair when you submit a program to the computer and it says “syntax error” - given that it *knows* the language and you are just learning it. It seems rude and cruel. You must remember that you are intelligent and *can* learn - the computer is simple and very fast - but cannot learn - so it is easier for you to learn Python than for the computer to learn English...

30. The Essence of Programming

31. Software Software InputDevices CentralProcessingUnit MainMemory OutputDevices SecondaryMemory Generic Computer Z-5

32. Program Steps or Program Flow Like a recipe or installation instructions, a program is a sequence of steps to be done in order Some steps are conditional - they may be skipped Sometimes a step or group of steps are to be repeated Sometimes we store a set of steps to be used over and over as needed several places throughout the program Z-14

33. Sequential Steps Program: x = 1 print x x = x + 1 print x Output: 1 2 x = 1 print x x = x + 1 print x When a program is running, it flows from one step to the next. We as programmers set up “paths” for the program to follow.

34. Conditional Steps Output: Smaller Finis Program: x = 5 if x < 10: print "Smaller“ if x > 20: print "Bigger" print “Finis” x = 5 X < 10 ? print “Smaller” X > 20 ? print “Bigger” Z-199 print “Finis” Yes

35. Repeated Steps Output: 0 1 2 3 4 Bye Program: for i in range(5) : print i print “Bye” i = 0.. 4 print i Z-233 Loops (repeated steps) have iteration variables that change each time through a loop. Often these iteration variables go through a sequence of numbers. Done print “Bye”

36. Stored (and reused) Steps Output: Hello Fun Zip Hello Fun Program: def hello(): print "Hello" print "Fun" hello() print “Zip ”hello() def print “Hello” print “Fun” hello() print “Zip” We call these little stored chunks of code “subprograms” or “functions”. hello(): hello()

37. A Python Program Z-14

38. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() $ python chaos.pyThis program illustrates a chaotic functionEnter a number between 0 and 1: 0.6 0.936 0.2336256 0.698274248196 0.821680557759 0.571434313164 0.955098841721 0.16725167263 0.543186347468 0.96772626363 0.121805355011

39. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() Stored steps Calling the stored steps

40. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() Output Input

41. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() Repeated Code

42. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() x = 3.9 * x * (1 - x)print x

43. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() The colon (:) starts a block of indented code Indented code continues until a line is encountered that is less indented. Start End

44. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() The colon (:) starts a block of indented code Indented code continues until a line is encountered that is less indented. Start End

45. Variables and Assignment Statements Z-16

46. Variables and Assignments A variable is a scratch local to store some value such as a number or a string An assignment statement consists of an expression on the right hand side and a variable to store the result x = 3.9 * x * ( 1 - x )

47. def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): x = 3.9 * x * (1 - x) print x main() Assignment Statement into the variable named x

48. x = 3.9 * x * ( 1 - x ) 0.6 x Left side is an expression. Once expression is evaluated, the result is placed in (assigned to) x. 0.6 0.4 0.93 A variable is a memory location used to store a value (0.6).

49. x = 3.9 * x * ( 1 - x ) 0.6 0.93 x Right side is an expression. Once expression is evaluated, the result is placed in (assigned to) the variable on the left side (i.e. x). 0.93 A variable is a memory location used to store a value. The value stored in a variable can be updated by replacing the old value (0.6) with a new value (0.93).

50. Comments in Python Z-14

51. Comments in Python Anything after a # is ignored by Python Why comment? Describe what is going to happen in a sequence of code Document who wrote the code or other ancillary information Turn off a line of code - perhaps temporarily Z-14

52. # File: chaos.py # A simple program illustrating chaotic behavior def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range(10): # print i x = 3.9 * x * (1 - x) print x main() Comments can document our programs. Comments can also be used to temporarily turn off lines of code without deleting those lines in case we want them back later. Z-14

53. Summary This is a quick overview of Chapter 1 We will revisit these concepts throughout the course Focus on the big picture